1. Field of the Invention
This invention relates generally to the field of geophysical exploration for hydrocarbons. More specifically, the invention relates to a method of velocity correction in seismic processing and imaging.
2. Background of the Invention
A seismic survey is a method of imaging the subsurface of the earth by delivering acoustic energy down into the subsurface and recording the signals reflected from the different rock layers below. The source of the acoustic energy typically comes from a seismic source such as without limitation, explosions or seismic vibrators on land, and air guns in marine environments. During a seismic survey, the seismic source may be moved across the surface of the earth above the geologic structure of interest. Each time a source is detonated or activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at different locations on the surface by receivers. The recordings or traces are then combined to create a profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the receivers are generally laid out along a single straight line, whereas in a three-dimensional (3D) survey the receivers are distributed across the surface in a grid pattern. A 2D seismic line provides a cross sectional picture (vertical slice) of the earth layers as arranged directly beneath the recording locations. A 3D survey produces a data “cube” or volume that theoretically represents a 3D picture of the subsurface that lies beneath the survey area.
In the oil and gas industry, the primary goal of seismic exploration is locating subterranean features of interest within a very large seismic volume. Rock stratigraphic information may be derived through the analysis of spatial variations in a seismic reflector's character because these variations may be empirically correlated with changes in reservoir lithology or fluid content. Since the exact geological basis behind these variations may not be well understood, a common method is to calculate a variety of attributes from the recorded seismic data and then plot or map them, looking for an attribute that has some predictive value. Given the extremely large amount of data collected in a 3-D volume, methods of enhancing the appearance of subsurface features related to the migration, accumulation, and presence of hydrocarbons are extremely valuable in seismic exploration.
In geological areas where the topography of the seafloor is highly complex or rugose (e.g. seafloor canyons, seamounts, and/or valleys), the seismic velocity model that is derived from conventional velocity estimation often cannot fully account for velocity changes related to the topography. Consequently, there is a need for methods and systems for correcting velocity in the field of seismic processing and interpretation in areas of complex topography.